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introductionASAD ALI (2013-EE-20)

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Lightening ARRESTERS

Contents Introduction of Lightning ArrestersIntroduction of lightning Lightning protectionConstruction of Lightning ArrestersHow does Lightning Arresters works?Uses of Lightning ArrestersTypes of Lightning Arresters

Lightning means very fast.

Arrester means policeman.

Introduction of LIGHTNING Arresters Alightning arresteris a device used on electrical power systems and telecommunications systems to protect the insulation and conductors of the system from the damaging effects oflightning. The typicallightning arresterhas a high-voltage terminal and a ground terminal.

What is Voltage Surge? A sudden rise in voltage for a very short duration on the power system is known as a voltage surge or transient voltage.

Other devices similar to lightening arrestersSurge Suppressor: This is also a surge diverter, but generally for voltages well below 1000 volts.TVSS (Transient Voltage Surge Suppressor): Again this is also a surge diverter, but generally for voltages well below 1000 volts.

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How do Lightning Arresters Protect Power Systems?

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Lightening Definition of Lightening Significance of Lightening Mechanism of lightening discharge Types of Lightening Effect of Lightening Protection against Lightening.

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Introduction of lightening What is Lightening?An electric discharge between cloud and earth, between clouds or between the charge centers of the same cloud is known as lightning. Lightening is a huge spark that take place when cloud is charged to such a high potential that dielectric strength of air is destroyed.

Introduction of lightening transmission line without surge arresterstransmission line with surge arresters

Mechanism of lightening dischargeLightening discharge is initiated by a streamer from the cloud which progress towards the earth in series of steps called Stepped Leader.Streamer moves from earth towards cloud is called Return Streamer.Leader streamer and return streamer has opposite charge.When Leader streamer and return streamer meets one another then a short circuit is established between earth and cloud give rise to a large current flow and Lightening phenomenon produced .

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Negative stepped leader

Positive answering leaders

Return stroke

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key points about lightening dischargeA lightning discharge usually made up of a number of separate strokes. 87% of all lightening strokes results from negatively charged clouds.13% of all lightening strokes results from positively charged clouds.Through out the world nearly 100 lightening strokes occur per secondLightening discharge current range from 10kA to 90kA.

Types of lightening strokesAlightning strokeis dened as a direct stroke if it hits either the tower or the shield wire or the phase conductor.Cloud will induce a charge of opposite polarity on lines when potential b/w cloud and line exceeds the breakdown value of air discharge occurs.

Direct stroke

Types of lightening strokesIndirect stroke results from the electrostatically induced charge on the conductor due to presence of charged clouds Induced charge leaks slowly to earth via insulators.Induced charge moves in both direction in the form of travelling waves. Most of the surge in transmission line is caused by indirect stroke.Indirect stroke

Effects of lighteningTraveling waves Insulator damageTransformer damageLightening discharge constitute the main cause of non-programmed outages of electrical system.65% line outages originate from Lightening strokes Surges produced by Lightening are not felt by most of the industrial equipment e.g. computers, Aluminum and steel mills .

Effects of lightening

Protection against lighteningEarthing screen.Overhead ground wiresLightening Arresters

Earthing screen

Earthing screenEarthing screen provides protection to power stations and sub-stations against direct stroke.It consist of a network of copper conductor mounted all over the electrical equipment in the substation and power station.Shield is properly ground through low resistance path.Limitation: It does not provide protection against travelling waves.

Over head ground wires Protect transmission line against direct stroke.Ground wires are placed above line conductors in such a way that all lightening strokes are intercepted by them.Tower rises potential is given by Vt= I.RVt less than insulator flashover then no trouble occurTower footing resistance must be very low.

Over head ground wires

Lightning arresters Provide protection against direct as well as indirect strokeProtect power station and Transmission line

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Earthing in view of lightening protectionTower Earthing resistance and surge impedance is an important parameter in determination of lightening flashover.Earthing resistance is a variable quantity.Depend upon soil and non linear conduction of earth Even if constant stroke current pass it changes with time.

contentsLightening RodClasses of Surge ArrestersWorking of lightening ArrestersSurge arrester in TransformerSurge arrester in Transmission line.Short circuit testWhat is MOV?How does dose current flows in Arresters

Lightening rodAlightning rodorlightning conductoris ametalrod or metallic object mounted on top of an elevated structure, such as abuilding, aship, or even atree,electrically bondedusing a wire orelectrical conductorto interface withground or "earth"through an electrode, engineered to protect the structure in the event oflightningstrike

Lightening rodSurge arresters is not a lightening Rod although it does the same operation that the lightening do.But its construction and working principle is very different from the lightening Rod.

Classes of surge arrestersStation ClassIntermediate ClassDistribution ClassSecondary Class

Working of surge arrestersNormal OperationOccurrence of High VoltageFunction as a non linear resistor

Surge arresters in transformerSurge Arresters are used in transformer both at transmission level and at distribution level.The rating of arresters used at these are different because of voltage levels.

Surge arresters in transformer

Surge arresters in transmission lineSurge Arrester is also used in Transmission system just for the same and basic purpose which is the safety of our equipment's and also the transmission line

Short circuit test of arresterBefore the installation of surge arrester many test are applied on this to mark it as verified.One of them is short circuit testIn this surge arrester is introduced to very high voltage surge and check weather It can bear it or not.

Surge arresters (basic construction)Surge arresters is consist of MOV discs.What is MOV?MOV is Metal Oxide Varistor.The most common type ofVaristoris theMetal-Oxide Varistor(MOV). This type contains a ceramic mass of zincoxidegrains, in a matrix of othermetal oxides(such as small amounts of bismuth, cobalt, manganese) sandwiched between twometalplates (the electrodes)

Surge arresters (basic construction)The basic structure of surge arresters is consist of MOV discs which basically acts as a semiconductor for normal voltages but when a surge appears or comes it becomes a conductor and provides a short path to the ground to a very high voltage surge.

Metal Oxide Discs

The mov diskThe MOV disk is a very fast acting electronic switchIt is an open switch to standard system AC voltage and a close switch to lightening voltages.By magnifying the MOV material 5000 times, Metal Oxide Grains and Dopants in the material can be discernedEach MOV Disk with a 35mm diameter and a 35mm height contains about 28 Billion MOV Grains

The MOV Grains and their Junctions are the Electronic Switches that turn on and off in unison to divert the lightning around the equipment.

A lightning arrester is essentially a collection of billions of microscopic junctions of Metal Oxide Grains that turn on and off in microseconds to form a current path from the top terminal to the ground terminal of the arrester.

So there you have it. A Lightning Arrester is a device, used on power systems, that contains billions of electronic switches that divert lightning around sensitive equipment and saves them from damage.

Types of Surge ArrestersRod Gap ArresterHorn Gap ArresterMultigap ArrestersExpulsion Type ArresterValve Type Arresters

Rod Gap ArresterSimple type of diverter.Consists of two 15 cm rods.The distance between gap and insulator must not be less than one-third of the gap length.Due to its limitations, Rod-gap arresters are used only as Back up protection.

Advantages Disadvantages Under normal operating conditions gap remains non-conducting.On the occurrence of a high voltage surge on the line, the gap sparks over and the surge current is conducted to earth.

Rods may melt or get damaged.The climatic conditions affect the performance of rod gap arrester.The polarity of the surge also affects the performance of this arrester.

Horn Gap ArrestersConsists of two horn shaped metal.Horn is connected to the line through a resistance R and choke coil L.Choke does not allow the transients to enter the apparatus to be protected.

Advantages disadvantagesThe arc is self-clearing.Series resistance helps in limiting the follow current to a small value.The bridging of gap can render the device useless.The setting of horn gap is likely to change due to corrosion or pitting.Time of operation is comparatively long.

multigap arresterIt consists of a series of metallic (generally alloy of zinc) cylinders.Cylinders are separated by air gap and are connected with series and shunt resistances.Such arresters are applied where voltage does not exceed 33kV.

Advantages DisadvantagesThey have the lowest let-through voltage and the best controlled clamping behavior of the various arrestor types.It provides safety and prevents damage to expensive equipment.

Their current handling capabilities are the smallest of all.Such arresters can be employed only where system voltage may not exceed 33kV.

Expulsion Type ArrestersThis type of arrester is also called Protector tube.Commonly used on system operating at voltages up to 33 kV.It essentially consists of Rod Gap, Upper Electrode and Lower Electrode.One expulsion arrester is placed under each line conductor.

Advantages disadvantagesThey are not very expensive.They are improved form of rod gap arresters.They can be easily installed. They can perform only limited number of operations.it is not suitable for the protection of expensive equipment. This type of arresters cannot be mounted in an enclosed equipment.

Valve type arresters

They are extensively used on systems operating at high voltages. They incorporate non-linear resistorsIt consists of two assemblies Series spark gaps Non-linear resistor discs.The non linear elements are connected in series with the spark gap.

Advantages DisadvantagesThey provide very effective protection (especially for transformers and cables) against surges. They operate very rapidly They fail to check the surges of every steep Wave.Their performance is adversely affected by the entry of moisture into the enclosure.

Applications of valve type arrestersAccording to there applications valve type arresters are classified as Two typesStation type: Generally used for protection of equipment in power stations operating on voltages upto 220kV or higher. Line type: Are used for station handling voltages upto 66kV.

A short video why a lightning arrester is necessary on transmission line?

Thank You.